Television receivers, both monochrome and color, respond to a composite signal which includes horizontal and vertical synchronizing signal pulses used by the television receiver to synchronize the beam deflection of the picture tube with the transmitted signal for producing accurate images. This is accomplished, in part, by producing accurately timed reference pulses in response to the received horizontal and vertical synchronizing signal components of the received signal. The horizontal synchronizing signal comprises a series of pulses at the line frequency, and the leading edge of each of these pulses occurs at the start of each line to be traced horizontally across the width of the picture tube. In the standard NTSC signal used in the United States, these horizontal pulses are approximately five microseconds wide with a 63.5 microsecond interval between each of the horizontal pulses. As is well known in the United States, 525 lines comprise the number of lines in the picture. In other countries of the world similar signals are used but in some countries 625 lines comprise the picture instead of the 525 lines used in the United States. In all other respects the composition of the transmitted television signals are substantially the same.
The vertical synchronizing signals which occur at the commencement of each picture field comprise a group of six pulses, each broad in comparison with the horizontal line synchronizing pulses and occuring at a repetition rate which is equal to two times the line frequency. As a consequence, a complete vertical synchronizing signal occupies a time period equivalent to three lines. The signal is repeated for each field with the leading edge of the first of the six pulses occuring at the commencement of the field. Typically the width of the vertical synchronization pulses is almost equal to one-half a horizontal line duration so that these pulses are on the order of 27.3 microseconds wide.
The vertical synchronizing signal is preceded by a group of six narrow pulses called equalizing pulses which also occur at a repetition rate of twice the line frequency. A second group of six narrow equalizing pulses follows the vertical synchronizing signal. The pulse width of the equalizing pulses typically is 2.5 microseconds.
The horizontal and vertical synchronizing signals, along with the equalizing pulses, comprise a combined synchronizing signal. In the NTSC system, interlaced scanning is employed. This means that each complete picture comprises two fields which are referred to as the even and odd fields. The even-even fields commence at the beginning of the first line and contain half the total number of lines ending at the mid point of a line since the total number of lines is odd. The odd fields commence at the midpoint of the line on which the even fields end and contain the remaining lines, ending at the end of the last line of the picture.
The nature of the vertical synchronizing and the equalizing signals is such that they contain leading edges of pulses at times which correspond to the start of each line which occur when these signals of the composite synchronizing signal are present. As a result, horizontal synchronizing signals for each line readily are derived from the combined synchronizing signal even during those portions where the equalizing pulses and vertical synchronizing pulses are present.
The conventional method of vertical synchronization in most television receivers is to pass this composite synchronization signal through an integrater circuit and to use the signal output of the integrater circuit to trigger or synchronize the operation of a blocking oscillator. As long as the vertical sweep of the television picture tube is started in exactly the same point of the vertical synchronization pulse interval of the alternating odd and even fields, and as long as the sweep has equally good linearity for both fields, there is no interlace problem in the received picture. This however, is difficult to obtain with such a standard integrating circuit type of vertical synchronization. In general the slope of the integrated pulse produced by such circuits is selected to be shallow enough to provide immunity against false starts by horizontal pulses, noise "spikes" and the like. This provides good noise immunity but a shallow slope can result in and does result in timing errors caused by the residual firing level voltage ripples (resulting from insufficient power supply isolation or filtering). As a consequence the reproduced picture has "pairing" of horizontal lines of sequential picture fields instead of the desired 50%-50% interlace.
In the past, using integrater type vertical synchronization circuits, efforts to eliminate the undesired lack of interlace necessarily involved an undesired trade off with the good noise immunity since improved interlace only could be obtained from an integrater circuit by increasing the slope of the integrated pulse. This is accomplished by increasing the band width of the low pass filter of the integrater circuit which results in a directly proportional degradation of the noise immunity of the vertical synchronization circuit.
To overcome the disadvantages of integrater type synchronization circuits, systems have been proprosed which rely on counting the number of horizontal synchronization pulses and dividing these pulses by a fixed ratio to achieve the improved interlace or stability. Systems of this type, however, are not compatible with non standard broadcast signals or many consumer type video tape recorders (VTR) or video game systems. In addition, the acquisition time of such systems of the prior art using a "count down" technique is slower than the acquisition time of conventional integrater circuit type vertical synchronization systems.
Other approaches have been proposed which rely upon stable phase locked vertical oscillators. Such oscillators have desirable stability and obviously inherently possess good interlace capabilities but systems of this type have slow acquisition times as a result of the narrow and stable acquisition range. Consequently, such phase locked looped systems often are not compatible with non standard signal formats with which television receivers increasingly must be compatible.
U.S. Pat. No. 3,619,497 issued to Richard Ellis on Nov. 9, 1971 discloses a digital synchronization signal separater including a pair of cascaded multivibrators and coincidence gates for obtaining vertical synchronization pulses with fairly constant and accurately known timing with respect the field datum point. While the system disclosed in this patent provides the desired 50/50 interlace, it suffers from inherently poorer noise immunity than the conventional integrated circuit type of vertical synchronization signal separater circuit. Consequently the circuit tends to become disabled under noisy signal conditions which would not disable a conventional integrating circuit type of synchronizing signal separater.
Accordingly, it is desirable to provide a vertical synchronizing signal separater circuit which has optimum interlace and optimum noise immunity. This is particularly important for large screen television receivers such as projection television receivers, since poor interlace is much more apparent to a viewer watching such a receiver than when a small screen television receiver is being watched. Ideally it is desirable to provide a synchronizing signal separater circuit having good interlace and good noise immunity to such an extent that the conventional vertical "hold" control could be eliminated from the receiver.